ASCE 7-10 Section 12.8.7 P-Delta Effects vs AISC 2

[StrEng007]

Can anyone point out the difference/limitation between the P-Delta requirements from ASCE 7-10 Section 12.8.-16 vs the stability requirements of AISC 360-10 Appendix 7.

If you satisfy the θ ≤ 0.1, per ASCE, and you're allowed to ignore P-Delta, what does that mean for AISC Appendix 7? Is ASCE 7-10 putting separate requirements on the structure? So we need to do two separate P-Delta analysis?

 

[driftLimiter]

I think that Appendix 8 of AISC has more to do with the P-Delta Effects mentioned by ASCE 7. AISC has required forces that must include second order effects from whatever source of deflection. The ASCE 12.8.7 is specifically talking about seismic displacements and second order effects from those. ASCE 7 is saying if you have drifts it may increase the story shear you get from first order analysis, the is a global effect of P-delta, elements also have second order effects that may impact their strength.

 

[JoshPlumSE]

Excellent question.... That's one that I've had in the past. I was at an AISC task committee meeting and was talking with someone from the ASCE committee about those provisions and how they worked together (or didn't work together).
My thoughts / recollections may not be perfect, because this was a number of years ago now:
1) The purpose of the AISC provision is primarily related to the STABILTY design of the steel members themselves. So, those provisions apply regardless of what ASCE 12.8.7 says. This is why we see stuff in there about Tau_beta reducing the flexural stiffness of members used in the analysis. Essentially, accounting for the effect of "inelasticity" of highly loaded columns on the P-Delta analysis and overall stability of the structure.

2) The purpose of 12.8.7, on the other hand is mostly related to the post disaster stability and drift limits of the structure. There is a good amount of discussion of this in the commentary of ASCE-7.

3) In conclusion, the provisions are there for slightly different reasons. Reasons that are both related to P-Delta and stability, but which apply differently.

4) So, what do I do as an engineer? I run all my design checks and analysis first considering the AISC provisions only. The is a more rigorous set of requirements for stability of the member and the structure.... for the most part. Though, I will check the maximum 2nd order to 1st order drift ratio. Trying to keep this under about 1.25 (which would correspond to the maximum allowed by ASCE-7).

When I'm essentially done with this part (member design and steel design), I then go back and run the ASCE provisions. This would be using a P-Delta analysis, but probably using a Tau_b = 1.0 for the steel members. That will give me a good idea of the drift of the structure to compare against ASCE-7 limits, and checking the ASCE "stability ratio" (Theta) against the ASCE maximum value (0.25). Since I checked the maximum 2nd order / 1st order drift ratio during my steel design, I should pass this without a problem.

 

[Deker]

Although this paper compares the ASCE 7 requirements to the AISC direct analysis method rather than Appendix 7, it addresses the heart of your question: Link.

 

[JoshPlumSE]

When you start talking about dynamic effects (i.e. response spectra analysis or fundamental period) then the question becomes a little more complicated.

But, I will assume that we're talking about static analysis and the equivalent lateral force procedures.